1. Technical Field
This disclosure relates generally to analog television signal receivers, and, more specifically, to measuring video signal-to-noise ratio in analog television signals.
2. Description of the Related Art
Analog television (TV) signal receivers are generally configured to down-convert conventional radio frequency (RF) analog television signals to an intermediate frequency, and, in some cases, further demodulate to a baseband video signal. In this context, variations in video signal-to-noise ratio (VSNR) may cause artifacts and distortions in displayed video. Thus, it is generally desirable to maintain a consistent VSNR in the presence of RF blockers. In other words, rather than attempting to provide a highest attainable VSNR which may vary over time it is often better to provide a lower, consistent VSNR. In addition, some standards (CENELEC 55020, for example) define limits on VSNR degradation in the presence of RF blockers. Therefore, it may be desirable to suppress VSNR to a level that can be consistently maintained.
For example, when a TV receiver is receiving video data from a particular channel, signals from nearby TV channels or signals from other electronic devices (both examples of RF blockers) may cause VSNR variations. To solve this problem, an automatic gain control system in a receiver may vary an RF gain based on VSNR so as to optimize the noise performance of the receiver while preventing compression of the combined received desired signal and blocker in the receiver RF circuitry. To do so, a receiver may need to obtain the VSNR of the received video signal. TV receivers may also use VSNR data for other purposes.